This comes up regularly on the UK Model Engineer board.
You have to think as if you are the machine tool manufacturer making machines for a world market. Whichever units system you work in, it is more efficient in terms of stock inventory and tool inventory to use the same diameter stock for your leadscrews whether they are imperial or metric and just to vary the pitch of the thread cut on them (keeping the form/thread angle the same).
It also gives you an agile capability - to switch from making an imperial leadscrew to a metric one, just flip a lever to change the gearing of the machine.
No need to change the stock, the workholding for the stock or the cutting tool. No chance of mixing up imperial stock with metric stock. No chance of picking up the wrong toolbit. No need for the tool grinding department to fixture for 29 and 30 degrees.
You can do the same mental exercise for the nut with which the leadscrew works. Same stock, same tapping drill, only the tap itself changes. It might not conform to any conventional standard, but it makes very good economic sense.